Support Tempest config file format transition

* Identity server location just configure by an URI
* Image service location resolved by service endpoint
* Credentials will be defined only in the identity section
* Whitebox gets it's own section
* ssh username is per image option

Change-Id: I3b0d51a323560451c5636363896cadb39e0ea2d6
Merge "Support Tempest config file format transition"